1. Field of the Invention
This invention relates generally to a spinal implant system, and more particularly, to an interbody spinal fusion device for promoting the fusion of two adjacent vertebrae.
2. Description of the Related Art
There is a need in the field of neurosurgery to have spinal implant systems that allow surgeons to increase the probability of successful vertebral fusion procedures. Intervertebral discs that become degenerated due to various factors such as trauma, aging, or disease may be partially or fully removed as a method of pain control. In a process that is referred to as “interbody fusion,” bone graft material is placed into the intervertebral space where the disc was removed to enable adjacent vertebrae to grow together and become one solid piece of bone.
Fusion may require weeks, sometimes months, to achieve a desirable result. If relative movement takes place between the adjacent vertebrae while fusion is underway there is a risk of unsatisfactory results, and at a minimum, the rate of fusion will be retarded. Relative movement may also cause back pain after surgery. In addition, it is necessary to support the spine during fusion in order to maintain the height of the spine (i.e, intervertebral spacing) and, preferably also to maintain the normal lordosis of the spine.
Therefore, once the intervertebral disc is removed, an implant device is typically inserted between neighboring vertebrae to maintain normal disc spacing and restore spinal stability while facilitating intervertebral fusion.
Known implant devices for facilitating fusion include a threaded spinal implant comprising a hollow externally-threaded cylinder into which bone chips or slurry is placed. The cylinder is inserted into the intervertebral space and has holes extending radially therethrough so that bone material grows though the holes to fuse with the bone material of the vertebrae. Illustrative devices of this type are described in U.S. Pat. Nos. 5,947,971; 5,522,899; 5,489,308; 5,015,247; and 4,961,740. Commercial devices of this type include the Sulzer Spine-Tech, Inc. BAK™ surgical implant.
One problem with the implant devices of the type mentioned above is that they tend not to maintain the normal lordosis of the spine. In a healthy state, the cervical and lumbar areas of the human spine are lordotic such that they curve convexly forward. Normal lordosis results, at least in significant measure, from the normal wedge-shaped nature of the spaces between adjacent pairs of the cervical and lumbar vertebrae, and the normal wedge-shaped nature of the intervertebral discs that fill these spaces. Loss of lordosis and proper intervertebral spacing may result in an increased risk of degeneration to other intervertebral discs located adjacent to the fusion level due to the alteration of the overall mechanics of the spine. There is, therefore, a need for an interbody spinal fusion device that maintains normal disc spacing and lordosis.
As a result of the need to maintain proper intervertebral spacing, in this known arrangement, it is necessary to have multiple implant devices, in varying sizes, available for any given operation. For example, the Sulzer Spine-Tech, Inc. BAK™ system includes four different sizes of threaded implants as well as multiple varieties of other implements. This, of course, greatly increases the cost of the implant.
A further problem with the implant mentioned above is that the cylindrical geometry of the engaging element tends to provide a small area of contact between the engaging element and the vertebrae. The small engaging surface tends to contribute to subsidence or deformation of the cortical layer of the vertebrae adjacent to the engaging element. Moreover, the small engaging surface provides less contact between the bone graft material encased in the device and the adjacent vertebrae. Exposure of the bone graft material to the surface of the vertebrae is important because the more exposure, the greater the possibility of having fusion occur. There is, therefore, a need for an interbody spinal fusion device that permits an increased area of exposure of bone graft material to the adjacent vertebrae.
In addition to the foregoing, placement of the known devices is difficult and can shift as a result of procedures during the operation and after the operation. Further, despite the tendency of the threaded exterior to grip the endplates of adjacent vertebrae, the generally cylindrical character of these implant devices can permit relative movement of the vertebrae to take place.
Another type of known implant device is a cage element that has two engaging plates that fit into the intervertebral region. The height between the two plates is then adjusted by some mechanism so that the top plate rests firmly against the upper vertebrae and the lower engaging plate rests firmly against the lower vertebrae. Bone graft material is then inserted in to the cage element. The top and bottom engaging plates have holes that allow the bone graft material to come in contact with the vertebral surface. The cage element differs from the externally-threaded cylinder implant discussed above because the cage element has more surface area that is in contact with the adjacent vertebrae. However, the holes in the top and bottom plates are the only exposure that the bone graft material has to the vertebral surface. In this regard, the cage element is not significantly better that the externally-threaded cylinder implant.
Further, the cage element rests against the softer cancellous bone in the center of the vertebral bodies and/or is provided with protrusions or teeth to facilitate engagement with the cancellous bone tissue. The cage element is, therefore, not attached to the harder, outer ring of cortical bone. Moreover, engagement of the device to the cancellous bone surface causes damage that can result in subsidence.
There is, therefore, a need in the art for an interbody spinal fusion device that decreases the risk of subsidence and provides a larger area of contact between bone graft material and adjacent vertebrae. There is also a need in the art for an interbody spinal fusion device that maintains normal lordosis.